diff --git a/Cabal-QuickCheck/src/Test/QuickCheck/Instances/Cabal.hs b/Cabal-QuickCheck/src/Test/QuickCheck/Instances/Cabal.hs
index 0cdff2283e9..345f9981fea 100644
--- a/Cabal-QuickCheck/src/Test/QuickCheck/Instances/Cabal.hs
+++ b/Cabal-QuickCheck/src/Test/QuickCheck/Instances/Cabal.hs
@@ -40,6 +40,8 @@ import Distribution.Types.VersionRange.Internal
 import Distribution.Utils.NubList
 import Distribution.Verbosity
 import Distribution.Version
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel)
+import Distribution.Types.OptimisationLevel (OptimisationLevel)
 
 import Test.QuickCheck.GenericArbitrary
 
diff --git a/Cabal-tree-diff/src/Data/TreeDiff/Instances/Cabal.hs b/Cabal-tree-diff/src/Data/TreeDiff/Instances/Cabal.hs
index f28af98b600..2e7cf79b306 100644
--- a/Cabal-tree-diff/src/Data/TreeDiff/Instances/Cabal.hs
+++ b/Cabal-tree-diff/src/Data/TreeDiff/Instances/Cabal.hs
@@ -17,20 +17,22 @@ import Distribution.Compiler                       (CompilerFlavor, CompilerId,
 import Distribution.InstalledPackageInfo           (AbiDependency, ExposedModule, InstalledPackageInfo)
 import Distribution.ModuleName                     (ModuleName)
 import Distribution.PackageDescription
-import Distribution.Simple.Compiler                (DebugInfoLevel, OptimisationLevel, ProfDetailLevel)
+import Distribution.Simple.Compiler                (ProfDetailLevel)
 import Distribution.Simple.InstallDirs
-import Distribution.Simple.InstallDirs.Internal
+import Distribution.Simple.InstallDirs.Internal    (PathComponent)
 import Distribution.Simple.Setup                   (HaddockTarget, TestShowDetails)
-import Distribution.System
+import Distribution.System                         (OS, Arch)
 import Distribution.Types.AbiHash                  (AbiHash)
 import Distribution.Types.ComponentId              (ComponentId)
 import Distribution.Types.DumpBuildInfo            (DumpBuildInfo)
-import Distribution.Types.PackageVersionConstraint
+import Distribution.Types.PackageVersionConstraint (PackageVersionConstraint)
 import Distribution.Types.UnitId                   (DefUnitId, UnitId)
 import Distribution.Utils.NubList                  (NubList)
 import Distribution.Utils.Path                     (SymbolicPathX)
-import Distribution.Verbosity
-import Distribution.Verbosity.Internal
+import Distribution.Verbosity                      (VerbosityLevel, VerbosityFlags)
+import Distribution.Verbosity.Internal             (VerbosityFlag)
+import Distribution.Types.DebugInfoLevel           (DebugInfoLevel)
+import Distribution.Types.OptimisationLevel        (OptimisationLevel)
 
 import qualified Distribution.Compat.NonEmptySet as NES
 
diff --git a/Cabal/Cabal.cabal b/Cabal/Cabal.cabal
index 1faa5302923..a4a0f9f95d2 100644
--- a/Cabal/Cabal.cabal
+++ b/Cabal/Cabal.cabal
@@ -176,11 +176,13 @@ library
     Distribution.TestSuite
     Distribution.Types.AnnotatedId
     Distribution.Types.ComponentInclude
+    Distribution.Types.DebugInfoLevel
     Distribution.Types.DumpBuildInfo
     Distribution.Types.PackageName.Magic
     Distribution.Types.ComponentLocalBuildInfo
     Distribution.Types.LocalBuildConfig
     Distribution.Types.LocalBuildInfo
+    Distribution.Types.OptimisationLevel
     Distribution.Types.TargetInfo
     Distribution.Types.GivenComponent
     Distribution.Types.ParStrat
diff --git a/Cabal/src/Distribution/Simple/Build.hs b/Cabal/src/Distribution/Simple/Build.hs
index 1ceb75a4a54..8ac5cc2c4f0 100644
--- a/Cabal/src/Distribution/Simple/Build.hs
+++ b/Cabal/src/Distribution/Simple/Build.hs
@@ -304,7 +304,7 @@ dumpBuildInfo verbosity distPref dumpBuildInfoFlag pkg_descr lbi flags = do
     removeFileForcibly buildInfoFile
   where
     buildInfoFile = interpretSymbolicPathLBI lbi $ buildInfoPref distPref
-    shouldDumpBuildInfo = fromFlagOrDefault NoDumpBuildInfo dumpBuildInfoFlag == DumpBuildInfo
+    shouldDumpBuildInfo = dumpBuildInfoFlag == Flag DumpBuildInfo
 
     -- \| Given the flavor of the compiler, try to find out
     -- which program we need.
diff --git a/Cabal/src/Distribution/Simple/Command.hs b/Cabal/src/Distribution/Simple/Command.hs
index 937662b1d33..11bfc6d6947 100644
--- a/Cabal/src/Distribution/Simple/Command.hs
+++ b/Cabal/src/Distribution/Simple/Command.hs
@@ -73,7 +73,6 @@ module Distribution.Simple.Command
   , reqArg'
   , optArg
   , optArg'
-  , optArgDef'
   , noArg
   , boolOpt
   , boolOpt'
@@ -274,15 +273,6 @@ optArg'
 optArg' ad mkflag showflag =
   optArg ad (succeedReadE (mkflag . Just)) ("", mkflag Nothing) showflag
 
-optArgDef'
-  :: Monoid b
-  => ArgPlaceHolder
-  -> (String, Maybe String -> b)
-  -> (b -> [Maybe String])
-  -> MkOptDescr (a -> b) (b -> a -> a) a
-optArgDef' ad (dv, mkflag) showflag =
-  optArg ad (succeedReadE (mkflag . Just)) (dv, mkflag Nothing) showflag
-
 noArg :: Eq b => b -> MkOptDescr (a -> b) (b -> a -> a) a
 noArg flag sf lf d = choiceOpt [(flag, (sf, lf), d)] sf lf d
 
diff --git a/Cabal/src/Distribution/Simple/Compiler.hs b/Cabal/src/Distribution/Simple/Compiler.hs
index fbf159f11a6..076e52eb9b5 100644
--- a/Cabal/src/Distribution/Simple/Compiler.hs
+++ b/Cabal/src/Distribution/Simple/Compiler.hs
@@ -48,14 +48,6 @@ module Distribution.Simple.Compiler
   , coercePackageDBStack
   , readPackageDb
 
-    -- * Support for optimisation levels
-  , OptimisationLevel (..)
-  , flagToOptimisationLevel
-
-    -- * Support for debug info levels
-  , DebugInfoLevel (..)
-  , flagToDebugInfoLevel
-
     -- * Support for language extensions
   , CompilerFlag
   , languageToFlags
@@ -92,22 +84,30 @@ module Distribution.Simple.Compiler
   , showProfDetailLevel
   ) where
 
-import Distribution.Compat.CharParsing
 import Distribution.Compat.Prelude
-import Distribution.Parsec
-import Distribution.Pretty
+import Distribution.Parsec (CabalParsing, Parsec (..), parsecToken)
+import Distribution.Pretty (prettyShow)
 import Prelude ()
 
 import Distribution.Compiler
 import Distribution.Package (PackageName)
-import Distribution.Simple.Utils
+import Distribution.Simple.Utils (lowercase, safeLast)
 import Distribution.Types.UnitId (UnitId)
 import Distribution.Utils.Path
-import Distribution.Version
-
-import Language.Haskell.Extension
+  ( CWD
+  , FileOrDir (..)
+  , Pkg
+  , PkgDB
+  , SymbolicPath
+  , getSymbolicPath
+  , interpretSymbolicPath
+  , makeSymbolicPath
+  , symbolicPathRelative_maybe
+  )
+import Distribution.Version (Version, mkVersion)
+
+import Language.Haskell.Extension (Extension, Language (..))
 
-import Data.Bool (bool)
 import qualified Data.Map as Map (lookup)
 import System.Directory (canonicalizePath)
 
@@ -300,95 +300,6 @@ coercePackageDBStack = map coercePackageDB
 
 -- ------------------------------------------------------------
 
--- * Optimisation levels
-
--- ------------------------------------------------------------
-
--- | Some compilers support optimising. Some have different levels.
--- For compilers that do not the level is just capped to the level
--- they do support.
-data OptimisationLevel
-  = NoOptimisation
-  | NormalOptimisation
-  | MaximumOptimisation
-  deriving (Bounded, Enum, Eq, Generic, Read, Show)
-
-instance Binary OptimisationLevel
-instance NFData OptimisationLevel
-instance Structured OptimisationLevel
-
-instance Parsec OptimisationLevel where
-  parsec = parsecOptimisationLevel
-
-parsecOptimisationLevel :: CabalParsing m => m OptimisationLevel
-parsecOptimisationLevel = boolParser <|> intParser
-  where
-    boolParser = bool NoOptimisation NormalOptimisation <$> parsec
-    intParser = intToOptimisationLevel <$> integral
-
-flagToOptimisationLevel :: Maybe String -> OptimisationLevel
-flagToOptimisationLevel Nothing = NormalOptimisation
-flagToOptimisationLevel (Just s) = case reads s of
-  [(i, "")] -> intToOptimisationLevel i
-  _ -> error $ "Can't parse optimisation level " ++ s
-
-intToOptimisationLevel :: Int -> OptimisationLevel
-intToOptimisationLevel i
-  | i >= minLevel && i <= maxLevel = toEnum i
-  | otherwise =
-      error $
-        "Bad optimisation level: "
-          ++ show i
-          ++ ". Valid values are "
-          ++ show minLevel
-          ++ ".."
-          ++ show maxLevel
-  where
-    minLevel = fromEnum (minBound :: OptimisationLevel)
-    maxLevel = fromEnum (maxBound :: OptimisationLevel)
-
--- ------------------------------------------------------------
-
--- * Debug info levels
-
--- ------------------------------------------------------------
-
--- | Some compilers support emitting debug info. Some have different
--- levels.  For compilers that do not the level is just capped to the
--- level they do support.
-data DebugInfoLevel
-  = NoDebugInfo
-  | MinimalDebugInfo
-  | NormalDebugInfo
-  | MaximalDebugInfo
-  deriving (Bounded, Enum, Eq, Generic, Read, Show)
-
-instance Binary DebugInfoLevel
-instance NFData DebugInfoLevel
-instance Structured DebugInfoLevel
-
-instance Parsec DebugInfoLevel where
-  parsec = parsecDebugInfoLevel
-
-parsecDebugInfoLevel :: CabalParsing m => m DebugInfoLevel
-parsecDebugInfoLevel = flagToDebugInfoLevel . pure <$> parsecToken
-
-flagToDebugInfoLevel :: Maybe String -> DebugInfoLevel
-flagToDebugInfoLevel Nothing = NormalDebugInfo
-flagToDebugInfoLevel (Just s) = case reads s of
-  [(i, "")]
-    | i >= fromEnum (minBound :: DebugInfoLevel)
-        && i <= fromEnum (maxBound :: DebugInfoLevel) ->
-        toEnum i
-    | otherwise ->
-        error $
-          "Bad debug info level: "
-            ++ show i
-            ++ ". Valid values are 0..3"
-  _ -> error $ "Can't parse debug info level " ++ s
-
--- ------------------------------------------------------------
-
 -- * Languages and Extensions
 
 -- ------------------------------------------------------------
diff --git a/Cabal/src/Distribution/Simple/Configure.hs b/Cabal/src/Distribution/Simple/Configure.hs
index 53c3b35caa0..a5adee3e755 100644
--- a/Cabal/src/Distribution/Simple/Configure.hs
+++ b/Cabal/src/Distribution/Simple/Configure.hs
@@ -157,6 +157,7 @@ import Distribution.Pretty
   )
 import Distribution.Simple.Errors
 import Distribution.Types.AnnotatedId
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel (..))
 import Distribution.Utils.Path
 import Distribution.Utils.Structured (structuredDecodeOrFailIO, structuredEncode)
 import System.Directory
diff --git a/Cabal/src/Distribution/Simple/GHC/Internal.hs b/Cabal/src/Distribution/Simple/GHC/Internal.hs
index 6959b8f48b6..5ad852fe742 100644
--- a/Cabal/src/Distribution/Simple/GHC/Internal.hs
+++ b/Cabal/src/Distribution/Simple/GHC/Internal.hs
@@ -72,11 +72,13 @@ import Distribution.Simple.Utils
 import Distribution.System
 import Distribution.Types.BuildInfo
 import Distribution.Types.ComponentLocalBuildInfo
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel (..))
 import Distribution.Types.GivenComponent
 import qualified Distribution.Types.InstalledPackageInfo as IPI
 import Distribution.Types.Library
 import Distribution.Types.LocalBuildInfo
 import Distribution.Types.ModuleRenaming
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
 import Distribution.Types.PackageName
 import Distribution.Types.TargetInfo
 import Distribution.Types.UnitId
diff --git a/Cabal/src/Distribution/Simple/Program/GHC.hs b/Cabal/src/Distribution/Simple/Program/GHC.hs
index 1b6a1857d11..caf8fe89886 100644
--- a/Cabal/src/Distribution/Simple/Program/GHC.hs
+++ b/Cabal/src/Distribution/Simple/Program/GHC.hs
@@ -45,12 +45,13 @@ import Distribution.Verbosity
 import Distribution.Version
 
 import GHC.IO.Encoding (TextEncoding)
-import Language.Haskell.Extension
+import Language.Haskell.Extension (Extension, Language)
 
 import Data.List (stripPrefix)
 import qualified Data.Map as Map
 import Data.Monoid (All (..), Any (..), Endo (..))
 import qualified Data.Set as Set
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel (..))
 import qualified System.Process as Process
 
 normaliseGhcArgs :: Maybe Version -> PackageDescription -> [String] -> [String]
diff --git a/Cabal/src/Distribution/Simple/Setup/Config.hs b/Cabal/src/Distribution/Simple/Setup/Config.hs
index 839241eed0f..fd052de41da 100644
--- a/Cabal/src/Distribution/Simple/Setup/Config.hs
+++ b/Cabal/src/Distribution/Simple/Setup/Config.hs
@@ -59,14 +59,19 @@ import Distribution.Simple.Program
 import Distribution.Simple.Setup.Common
 import Distribution.Simple.Utils
 import Distribution.Types.ComponentId
-import Distribution.Types.DumpBuildInfo
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel (..))
+import qualified Distribution.Types.DebugInfoLevel as D
+import Distribution.Types.DumpBuildInfo (DumpBuildInfo (..))
 import Distribution.Types.GivenComponent
 import Distribution.Types.Module
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
+import qualified Distribution.Types.OptimisationLevel as O
 import Distribution.Types.PackageVersionConstraint
 import Distribution.Types.UnitId
 import Distribution.Utils.NubList
 import Distribution.Utils.Path
 import Distribution.Verbosity
+import Text.Printf (printf)
 
 import qualified Text.PrettyPrint as Disp
 
@@ -434,7 +439,7 @@ configureOptions showOrParseArgs =
         configHcFlavor
         (\v flags -> flags{configHcFlavor = v})
         ( choiceOpt
-            [ (Flag GHC, ("g", ["ghc"]), "compile with GHC")
+            [ (Flag GHC, ([], ["ghc"]), "compile with GHC")
             , (Flag GHCJS, ([], ["ghcjs"]), "compile with GHCJS")
             , (Flag UHC, ([], ["uhc"]), "compile with UHC")
             ]
@@ -567,18 +572,16 @@ configureOptions showOrParseArgs =
           "optimization"
           configOptimization
           (\v flags -> flags{configOptimization = v})
-          [ optArgDef'
+          [ optArg'
               "n"
-              (show NoOptimisation, Flag . flagToOptimisationLevel)
+              (Flag . maybe NormalOptimisation fromString)
               ( \case
-                  Flag NoOptimisation -> []
-                  Flag NormalOptimisation -> [Nothing]
-                  Flag MaximumOptimisation -> [Just "2"]
-                  _ -> []
+                  NoFlag -> []
+                  Flag flag -> [Just $ O.toString flag]
               )
               "O"
               ["enable-optimization", "enable-optimisation"]
-              "Build with optimization (n is 0--2, default is 1)"
+              (printf "Build with optimization (n is %s--%s, default is %s)" (O.toString minBound) (O.toString maxBound) (O.toString NormalOptimisation))
           , noArg
               (Flag NoOptimisation)
               []
@@ -591,17 +594,14 @@ configureOptions showOrParseArgs =
           (\v flags -> flags{configDebugInfo = v})
           [ optArg'
               "n"
-              (Flag . flagToDebugInfoLevel)
+              (Flag . maybe NoDebugInfo fromString)
               ( \case
-                  Flag NoDebugInfo -> []
-                  Flag MinimalDebugInfo -> [Just "1"]
-                  Flag NormalDebugInfo -> [Nothing]
-                  Flag MaximalDebugInfo -> [Just "3"]
-                  _ -> []
+                  NoFlag -> []
+                  Flag flag -> [Just $ D.toString flag]
               )
-              ""
+              "g"
               ["enable-debug-info"]
-              "Emit debug info (n is 0--3, default is 0)"
+              (printf "Emit debug info (n is  %s--%s, default is %s)" (D.toString minBound) (D.toString maxBound) (D.toString NoDebugInfo))
           , noArg
               (Flag NoDebugInfo)
               []
diff --git a/Cabal/src/Distribution/Simple/UHC.hs b/Cabal/src/Distribution/Simple/UHC.hs
index bdea3d94fa1..e431e4c5f5a 100644
--- a/Cabal/src/Distribution/Simple/UHC.hs
+++ b/Cabal/src/Distribution/Simple/UHC.hs
@@ -41,6 +41,7 @@ import Distribution.Simple.Program
 import Distribution.Simple.Utils
 import Distribution.System
 import Distribution.Types.MungedPackageId
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
 import Distribution.Utils.Path
 import Distribution.Verbosity
 import Distribution.Version
diff --git a/Cabal/src/Distribution/Types/DebugInfoLevel.hs b/Cabal/src/Distribution/Types/DebugInfoLevel.hs
new file mode 100644
index 00000000000..036dd49948f
--- /dev/null
+++ b/Cabal/src/Distribution/Types/DebugInfoLevel.hs
@@ -0,0 +1,57 @@
+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE DerivingStrategies #-}
+
+module Distribution.Types.DebugInfoLevel
+  ( DebugInfoLevel (..)
+  , toString
+  )
+where
+
+import Distribution.Compat.CharParsing (integral)
+import Distribution.Compat.Prelude
+import Distribution.Parsec (CabalParsing, Parsec (..))
+import Prelude ()
+
+import Data.Bool (bool)
+
+-- | Some compilers support emitting debug info. Some have different
+-- levels.  For compilers that do not the level is just capped to the
+-- level they do support.
+data DebugInfoLevel
+  = NoDebugInfo
+  | MinimalDebugInfo
+  | NormalDebugInfo
+  | MaximalDebugInfo
+  deriving stock (Bounded, Enum, Eq, Generic, Read, Show)
+  deriving anyclass (Binary, NFData, Structured)
+
+instance Parsec DebugInfoLevel where
+  parsec :: CabalParsing m => m DebugInfoLevel
+  parsec = boolParser <|> intParser
+    where
+      boolParser = bool NoDebugInfo NormalDebugInfo <$> parsec
+      intParser = intToDebugInfoLevel <$> integral
+
+instance IsString DebugInfoLevel where
+  fromString :: String -> DebugInfoLevel
+  fromString s = case reads s of
+    [(i, "")] -> intToDebugInfoLevel i
+    _ -> error $ "Can't parse debug info level " ++ s
+
+toString :: DebugInfoLevel -> String
+toString = show . fromEnum
+
+intToDebugInfoLevel :: Int -> DebugInfoLevel
+intToDebugInfoLevel i
+  | i >= minLevel && i <= maxLevel = toEnum i
+  | otherwise =
+      error $
+        "Bad debug info level: "
+          ++ show i
+          ++ ". Valid values are "
+          ++ show minLevel
+          ++ ".."
+          ++ show maxLevel
+  where
+    minLevel = fromEnum (minBound :: DebugInfoLevel)
+    maxLevel = fromEnum (maxBound :: DebugInfoLevel)
diff --git a/Cabal/src/Distribution/Types/DumpBuildInfo.hs b/Cabal/src/Distribution/Types/DumpBuildInfo.hs
index 6b3bcb06488..0ffa303c507 100644
--- a/Cabal/src/Distribution/Types/DumpBuildInfo.hs
+++ b/Cabal/src/Distribution/Types/DumpBuildInfo.hs
@@ -1,24 +1,29 @@
+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE LambdaCase #-}
+
 module Distribution.Types.DumpBuildInfo
   ( DumpBuildInfo (..)
+  , toString
   ) where
 
-import Distribution.Compat.Prelude
-import Distribution.Parsec
+import Distribution.Compat.Prelude (Binary, Generic, NFData, Structured)
+import Distribution.Parsec (CabalParsing, Parsec (..))
 
 data DumpBuildInfo
   = NoDumpBuildInfo
   | DumpBuildInfo
-  deriving (Read, Show, Eq, Ord, Enum, Bounded, Generic)
-
-instance Binary DumpBuildInfo
-instance NFData DumpBuildInfo
-instance Structured DumpBuildInfo
+  deriving stock (Read, Show, Eq, Ord, Enum, Bounded, Generic)
+  deriving anyclass (Binary, NFData, Structured)
 
 instance Parsec DumpBuildInfo where
-  parsec = parsecDumpBuildInfo
-
-parsecDumpBuildInfo :: CabalParsing m => m DumpBuildInfo
-parsecDumpBuildInfo = boolToDumpBuildInfo <$> parsec
+  parsec :: CabalParsing m => m DumpBuildInfo
+  parsec = boolToDumpBuildInfo <$> parsec
 
 boolToDumpBuildInfo :: Bool -> DumpBuildInfo
 boolToDumpBuildInfo bool = if bool then DumpBuildInfo else NoDumpBuildInfo
+
+toString :: DumpBuildInfo -> String
+toString = \case
+  NoDumpBuildInfo -> "False"
+  DumpBuildInfo -> "True"
diff --git a/Cabal/src/Distribution/Types/LocalBuildConfig.hs b/Cabal/src/Distribution/Types/LocalBuildConfig.hs
index 984cbc18161..37ba5c701a8 100644
--- a/Cabal/src/Distribution/Types/LocalBuildConfig.hs
+++ b/Cabal/src/Distribution/Types/LocalBuildConfig.hs
@@ -17,25 +17,23 @@ module Distribution.Types.LocalBuildConfig
 import Distribution.Compat.Prelude
 import Prelude ()
 
-import Distribution.Types.ComponentLocalBuildInfo
-import Distribution.Types.ComponentRequestedSpec
-import Distribution.Types.GivenComponent
-import Distribution.Types.PackageDescription
-import Distribution.Types.UnitId
+import Distribution.Types.ComponentLocalBuildInfo (ComponentLocalBuildInfo)
+import Distribution.Types.ComponentRequestedSpec (ComponentRequestedSpec)
+import Distribution.Types.GivenComponent (PromisedComponent)
+import Distribution.Types.PackageDescription (PackageDescription)
+import Distribution.Types.UnitId (UnitId)
 
-import Distribution.PackageDescription
-import Distribution.Simple.Compiler
-import Distribution.Simple.Flag
-import Distribution.Simple.InstallDirs hiding
-  ( absoluteInstallDirs
-  , prefixRelativeInstallDirs
-  , substPathTemplate
-  )
-import Distribution.Simple.PackageIndex
+import Distribution.PackageDescription (ComponentName, FlagAssignment, PackageName)
+import Distribution.Simple.Compiler (Compiler, PackageDBStack, ProfDetailLevel)
+import Distribution.Simple.Flag (maybeToFlag, toFlag)
+import Distribution.Simple.InstallDirs (InstallDirTemplates, PathTemplate)
+import Distribution.Simple.PackageIndex (InstalledPackageIndex)
 import Distribution.Simple.Program.Db (ProgramDb)
-import Distribution.Simple.Setup.Config
-import Distribution.System
-import Distribution.Utils.Path
+import Distribution.Simple.Setup.Config (ConfigFlags (..))
+import Distribution.System (Platform)
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel)
+import Distribution.Types.OptimisationLevel (OptimisationLevel)
+import Distribution.Utils.Path (FileOrDir (..), Pkg, SymbolicPath)
 
 import Distribution.Compat.Graph (Graph)
 
diff --git a/Cabal/src/Distribution/Types/LocalBuildInfo.hs b/Cabal/src/Distribution/Types/LocalBuildInfo.hs
index 7e91a5ebeb2..14da1656c82 100644
--- a/Cabal/src/Distribution/Types/LocalBuildInfo.hs
+++ b/Cabal/src/Distribution/Types/LocalBuildInfo.hs
@@ -129,6 +129,8 @@ import Distribution.System
 import qualified Data.Map as Map
 import Distribution.Compat.Graph (Graph)
 import qualified Distribution.Compat.Graph as Graph
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel)
+import Distribution.Types.OptimisationLevel (OptimisationLevel)
 
 import qualified System.FilePath as FilePath (takeDirectory)
 
diff --git a/Cabal/src/Distribution/Types/OptimisationLevel.hs b/Cabal/src/Distribution/Types/OptimisationLevel.hs
new file mode 100644
index 00000000000..acd300a7a20
--- /dev/null
+++ b/Cabal/src/Distribution/Types/OptimisationLevel.hs
@@ -0,0 +1,56 @@
+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE DerivingStrategies #-}
+
+module Distribution.Types.OptimisationLevel
+  ( OptimisationLevel (..)
+  , toString
+  )
+where
+
+import Distribution.Compat.CharParsing (integral)
+import Distribution.Compat.Prelude
+import Distribution.Parsec (CabalParsing, Parsec (..))
+import Prelude ()
+
+import Data.Bool (bool)
+
+-- | Some compilers support optimising. Some have different levels.
+-- For compilers that do not the level is just capped to the level
+-- they do support.
+data OptimisationLevel
+  = NoOptimisation
+  | NormalOptimisation
+  | MaximumOptimisation
+  deriving stock (Bounded, Enum, Eq, Generic, Read, Show)
+  deriving anyclass (Binary, NFData, Structured)
+
+instance Parsec OptimisationLevel where
+  parsec :: CabalParsing m => m OptimisationLevel
+  parsec = boolParser <|> intParser
+    where
+      boolParser = bool NoOptimisation NormalOptimisation <$> parsec
+      intParser = intToOptimisationLevel <$> integral
+
+instance IsString OptimisationLevel where
+  fromString :: String -> OptimisationLevel
+  fromString s = case reads s of
+    [(i, "")] -> intToOptimisationLevel i
+    _ -> error $ "Can't parse optimisation level " ++ s
+
+toString :: OptimisationLevel -> String
+toString = show . fromEnum
+
+intToOptimisationLevel :: Int -> OptimisationLevel
+intToOptimisationLevel i
+  | i >= minLevel && i <= maxLevel = toEnum i
+  | otherwise =
+      error $
+        "Bad optimisation level: "
+          ++ show i
+          ++ ". Valid values are "
+          ++ show minLevel
+          ++ ".."
+          ++ show maxLevel
+  where
+    minLevel = fromEnum (minBound :: OptimisationLevel)
+    maxLevel = fromEnum (maxBound :: OptimisationLevel)
diff --git a/cabal-install/parser-tests/Tests/ParserTests.hs b/cabal-install/parser-tests/Tests/ParserTests.hs
index 25ada09ce64..5ea07821c23 100644
--- a/cabal-install/parser-tests/Tests/ParserTests.hs
+++ b/cabal-install/parser-tests/Tests/ParserTests.hs
@@ -31,7 +31,7 @@ import Distribution.Client.Types.WriteGhcEnvironmentFilesPolicy (WriteGhcEnviron
 import Distribution.Compat.Prelude
 import Distribution.Compiler (CompilerFlavor (..))
 import Distribution.Parsec (simpleParsec)
-import Distribution.Simple.Compiler (DebugInfoLevel (..), OptimisationLevel (..), PackageDBX (..), ProfDetailLevel (..))
+import Distribution.Simple.Compiler (PackageDBX (..), ProfDetailLevel (..))
 import Distribution.Simple.Flag
 import Distribution.Simple.InstallDirs (InstallDirs (..), toPathTemplate)
 import Distribution.Simple.Setup (DumpBuildInfo (..), HaddockTarget (..), TestShowDetails (..))
@@ -50,7 +50,9 @@ import Distribution.Solver.Types.Settings
   )
 import Distribution.System (OS (..), buildOS)
 import Distribution.Types.CondTree (CondTree (..))
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel (..))
 import Distribution.Types.Flag (mkFlagAssignment)
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
 import Distribution.Types.PackageId (PackageIdentifier (..))
 import Distribution.Types.PackageName
 import Distribution.Types.PackageVersionConstraint (PackageVersionConstraint (..))
diff --git a/cabal-install/src/Distribution/Client/Config.hs b/cabal-install/src/Distribution/Client/Config.hs
index 03c6fd87652..5ec73f8b4ec 100644
--- a/cabal-install/src/Distribution/Client/Config.hs
+++ b/cabal-install/src/Distribution/Client/Config.hs
@@ -1,5 +1,4 @@
 {-# LANGUAGE DerivingVia #-}
-{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE PatternSynonyms #-}
 
 -- |
@@ -161,10 +160,7 @@ import Distribution.Simple.Command
   , ShowOrParseArgs (..)
   , commandDefaultFlags
   )
-import Distribution.Simple.Compiler
-  ( DebugInfoLevel (..)
-  , OptimisationLevel (..)
-  )
+import Distribution.Simple.Flag (Flag, flagElim, flagToMaybe, fromFlagOrDefault, toFlag, pattern Flag, pattern NoFlag)
 import Distribution.Simple.InstallDirs
   ( InstallDirs (..)
   , PathTemplate
@@ -178,7 +174,6 @@ import Distribution.Simple.Setup
   ( BenchmarkFlags (..)
   , CommonSetupFlags (..)
   , ConfigFlags (..)
-  , Flag
   , HaddockFlags (..)
   , TestFlags (..)
   , configureOptions
@@ -186,16 +181,11 @@ import Distribution.Simple.Setup
   , defaultConfigFlags
   , defaultHaddockFlags
   , defaultTestFlags
-  , flagToMaybe
-  , fromFlagOrDefault
   , haddockOptions
   , installDirsOptions
   , optionDistPref
   , programDbOptions
   , programDbPaths'
-  , toFlag
-  , pattern Flag
-  , pattern NoFlag
   )
 import Distribution.Simple.Utils
   ( cabalVersion
@@ -207,6 +197,8 @@ import Distribution.Simple.Utils
   , writeFileAtomic
   )
 import Distribution.Solver.Types.ConstraintSource
+import qualified Distribution.Types.DebugInfoLevel as DebugInfoLevel
+import qualified Distribution.Types.OptimisationLevel as OptimisationLevel
 import Distribution.Utils.Path (getSymbolicPath, unsafeMakeSymbolicPath)
 import Distribution.Verbosity
   ( normal
@@ -1180,58 +1172,23 @@ configFieldDescriptions src =
           (Flag <$> parsec <|> pure NoFlag)
           configHcFlavor
           (\v flags -> flags{configHcFlavor = v})
-      , -- TODO: The following is a temporary fix. The "optimization"
-        -- and "debug-info" fields are OptArg, and viewAsFieldDescr
-        -- fails on that. Instead of a hand-written hackaged parser
-        -- and printer, we should handle this case properly in the
-        -- library.
-        liftField
-          configOptimization
-          ( \v flags ->
-              flags{configOptimization = v}
-          )
-          $ let name = "optimization"
-             in FieldDescr
-                  name
-                  ( \case
-                      Flag NoOptimisation -> Disp.text "False"
-                      Flag NormalOptimisation -> Disp.text "True"
-                      Flag MaximumOptimisation -> Disp.text "2"
-                      _ -> Disp.empty
-                  )
-                  ( \line str _ -> case () of
-                      _
-                        | str == "0" -> ParseOk [] (Flag NoOptimisation)
-                        | str == "1" -> ParseOk [] (Flag NormalOptimisation)
-                        | str == "2" -> ParseOk [] (Flag MaximumOptimisation)
-                        | lstr == "false" -> ParseOk [] (Flag NoOptimisation)
-                        | lstr == "true" -> ParseOk [] (Flag NormalOptimisation)
-                        | otherwise -> ParseFailed (NoParse name line)
-                        where
-                          lstr = lowercase str
-                  )
+      , liftField configOptimization (\v flags -> flags{configOptimization = v}) $
+          let name = "optimization"
+           in FieldDescr
+                name
+                (flagElim Disp.empty (Disp.text . OptimisationLevel.toString))
+                ( \line str _ -> case maybe NoFlag Flag (simpleParsec str) of
+                    NoFlag -> ParseFailed (NoParse name line)
+                    flag -> ParseOk [] flag
+                )
       , liftField configDebugInfo (\v flags -> flags{configDebugInfo = v}) $
           let name = "debug-info"
            in FieldDescr
                 name
-                ( \case
-                    Flag NoDebugInfo -> Disp.text "False"
-                    Flag MinimalDebugInfo -> Disp.text "1"
-                    Flag NormalDebugInfo -> Disp.text "True"
-                    Flag MaximalDebugInfo -> Disp.text "3"
-                    _ -> Disp.empty
-                )
-                ( \line str _ -> case () of
-                    _
-                      | str == "0" -> ParseOk [] (Flag NoDebugInfo)
-                      | str == "1" -> ParseOk [] (Flag MinimalDebugInfo)
-                      | str == "2" -> ParseOk [] (Flag NormalDebugInfo)
-                      | str == "3" -> ParseOk [] (Flag MaximalDebugInfo)
-                      | lstr == "false" -> ParseOk [] (Flag NoDebugInfo)
-                      | lstr == "true" -> ParseOk [] (Flag NormalDebugInfo)
-                      | otherwise -> ParseFailed (NoParse name line)
-                      where
-                        lstr = lowercase str
+                (flagElim Disp.empty (Disp.text . DebugInfoLevel.toString))
+                ( \line str _ -> case maybe NoFlag Flag (simpleParsec str) of
+                    NoFlag -> ParseFailed (NoParse name line)
+                    flag -> ParseOk [] flag
                 )
       ]
     ++ toSavedConfig
diff --git a/cabal-install/src/Distribution/Client/DistDirLayout.hs b/cabal-install/src/Distribution/Client/DistDirLayout.hs
index ca32684dd98..a182f01a091 100644
--- a/cabal-install/src/Distribution/Client/DistDirLayout.hs
+++ b/cabal-install/src/Distribution/Client/DistDirLayout.hs
@@ -44,7 +44,6 @@ import Distribution.Package
   )
 import Distribution.Simple.Compiler
   ( Compiler (..)
-  , OptimisationLevel (..)
   , PackageDBCWD
   , PackageDBStackCWD
   , PackageDBX (..)
@@ -53,6 +52,7 @@ import Distribution.Simple.Configure (interpretPackageDbFlags)
 import Distribution.System
 import Distribution.Types.ComponentName
 import Distribution.Types.LibraryName
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
 
 -- | Information which can be used to construct the path to
 -- the build directory of a build.  This is LESS fine-grained
diff --git a/cabal-install/src/Distribution/Client/PackageHash.hs b/cabal-install/src/Distribution/Client/PackageHash.hs
index 7fdb3a571e3..dd59e9af47c 100644
--- a/cabal-install/src/Distribution/Client/PackageHash.hs
+++ b/cabal-install/src/Distribution/Client/PackageHash.hs
@@ -39,8 +39,6 @@ import Distribution.Package
 import Distribution.Simple.Compiler
   ( AbiTag (..)
   , CompilerId
-  , DebugInfoLevel (..)
-  , OptimisationLevel (..)
   , PackageDBCWD
   , ProfDetailLevel (..)
   , showProfDetailLevel
@@ -55,10 +53,12 @@ import Distribution.System
   , Platform
   , buildOS
   )
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel (..))
 import Distribution.Types.Flag
   ( FlagAssignment
   , showFlagAssignment
   )
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
 import Distribution.Types.PkgconfigVersion (PkgconfigVersion)
 
 import qualified Data.ByteString.Lazy.Char8 as LBS
@@ -324,7 +324,7 @@ renderPackageHashInputs
           , opt "split-sections" False prettyShow pkgHashSplitSections
           , opt "stripped-lib" False prettyShow pkgHashStripLibs
           , opt "stripped-exe" True prettyShow pkgHashStripExes
-          , opt "debug-info" NormalDebugInfo (show . fromEnum) pkgHashDebugInfo
+          , opt "debug-info" NoDebugInfo (show . fromEnum) pkgHashDebugInfo
           , opt "extra-lib-dirs" [] unwords pkgHashExtraLibDirs
           , opt "extra-lib-dirs-static" [] unwords pkgHashExtraLibDirsStatic
           , opt "extra-framework-dirs" [] unwords pkgHashExtraFrameworkDirs
diff --git a/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs b/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs
index fe39c8a23e0..1ad94dd7b03 100644
--- a/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs
+++ b/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs
@@ -85,8 +85,6 @@ import Distribution.PackageDescription.Configuration (simplifyWithSysParams)
 import Distribution.Simple.Compiler
   ( Compiler (..)
   , CompilerInfo (..)
-  , DebugInfoLevel (..)
-  , OptimisationLevel (..)
   , compilerInfo
   , interpretPackageDB
   )
@@ -107,8 +105,6 @@ import Distribution.Simple.Setup
   ( BenchmarkFlags (..)
   , CommonSetupFlags (..)
   , ConfigFlags (..)
-  , DumpBuildInfo (DumpBuildInfo, NoDumpBuildInfo)
-  , Flag
   , HaddockFlags (..)
   , TestFlags (..)
   , benchmarkOptions'
@@ -124,13 +120,9 @@ import Distribution.Simple.Setup
   , showPackageDb
   , splitArgs
   , testOptions'
-  , toFlag
-  , pattern Flag
-  , pattern NoFlag
   )
 import Distribution.Simple.Utils
   ( debug
-  , lowercase
   , noticeDoc
   )
 import Distribution.Types.CondTree
@@ -141,6 +133,7 @@ import Distribution.Types.CondTree
   , mapTreeData
   , traverseCondTreeV
   )
+import qualified Distribution.Types.DumpBuildInfo as DumpBuildInfo
 import Distribution.Types.SourceRepo (RepoType)
 import Distribution.Utils.NubList
   ( fromNubList
@@ -196,6 +189,9 @@ import qualified Data.ByteString.Char8 as BS
 import Data.Functor ((<&>))
 import qualified Data.Map as Map
 import qualified Data.Set as Set
+import Distribution.Simple.Flag (Flag, flagElim, toFlag, pattern Flag, pattern NoFlag)
+import qualified Distribution.Types.DebugInfoLevel as DebugInfoLevel
+import qualified Distribution.Types.OptimisationLevel as OptimisationLevel
 import Network.URI (URI (..), nullURIAuth)
 import System.Directory (makeAbsolute)
 import System.FilePath (splitFileName)
@@ -1674,79 +1670,36 @@ legacyPackageConfigFieldDescrs =
         (\v flags -> flags{configHcFlavor = v})
 
     overrideDumpBuildInfo =
-      liftField
-        configDumpBuildInfo
-        (\v flags -> flags{configDumpBuildInfo = v})
-        $ let name = "build-info"
-           in FieldDescr
-                name
-                ( \case
-                    Flag NoDumpBuildInfo -> Disp.text "False"
-                    Flag DumpBuildInfo -> Disp.text "True"
-                    _ -> Disp.empty
-                )
-                ( \line str _ -> case () of
-                    _
-                      | lstr == "false" -> ParseOk [] (Flag NoDumpBuildInfo)
-                      | lstr == "true" -> ParseOk [] (Flag DumpBuildInfo)
-                      | otherwise -> ParseFailed (NoParse name line)
-                      where
-                        lstr = lowercase str
-                )
-
-    -- TODO: [code cleanup] The following is a hack. The "optimization" and
-    -- "debug-info" fields are OptArg, and viewAsFieldDescr fails on that.
-    -- Instead of a hand-written parser and printer, we should handle this case
-    -- properly in the library.
+      liftField configDumpBuildInfo (\v flags -> flags{configDumpBuildInfo = v}) $
+        let name = "build-info"
+         in FieldDescr
+              name
+              (flagElim Disp.empty (Disp.text . DumpBuildInfo.toString))
+              ( \line str _ -> case maybe NoFlag Flag (simpleParsec str) of
+                  NoFlag -> ParseFailed (NoParse name line)
+                  flag -> ParseOk [] flag
+              )
 
     overrideFieldOptimization =
-      liftField
-        configOptimization
-        (\v flags -> flags{configOptimization = v})
-        $ let name = "optimization"
-           in FieldDescr
-                name
-                ( \case
-                    Flag NoOptimisation -> Disp.text "False"
-                    Flag NormalOptimisation -> Disp.text "True"
-                    Flag MaximumOptimisation -> Disp.text "2"
-                    _ -> Disp.empty
-                )
-                ( \line str _ -> case () of
-                    _
-                      | str == "0" -> ParseOk [] (Flag NoOptimisation)
-                      | str == "1" -> ParseOk [] (Flag NormalOptimisation)
-                      | str == "2" -> ParseOk [] (Flag MaximumOptimisation)
-                      | lstr == "false" -> ParseOk [] (Flag NoOptimisation)
-                      | lstr == "true" -> ParseOk [] (Flag NormalOptimisation)
-                      | otherwise -> ParseFailed (NoParse name line)
-                      where
-                        lstr = lowercase str
-                )
+      liftField configOptimization (\v flags -> flags{configOptimization = v}) $
+        let name = "optimization"
+         in FieldDescr
+              name
+              (flagElim Disp.empty (Disp.text . OptimisationLevel.toString))
+              ( \line str _ -> case maybe NoFlag Flag (simpleParsec str) of
+                  NoFlag -> ParseFailed (NoParse name line)
+                  flag -> ParseOk [] flag
+              )
 
     overrideFieldDebugInfo =
       liftField configDebugInfo (\v flags -> flags{configDebugInfo = v}) $
         let name = "debug-info"
          in FieldDescr
               name
-              ( \case
-                  Flag NoDebugInfo -> Disp.text "False"
-                  Flag MinimalDebugInfo -> Disp.text "1"
-                  Flag NormalDebugInfo -> Disp.text "True"
-                  Flag MaximalDebugInfo -> Disp.text "3"
-                  _ -> Disp.empty
-              )
-              ( \line str _ -> case () of
-                  _
-                    | str == "0" -> ParseOk [] (Flag NoDebugInfo)
-                    | str == "1" -> ParseOk [] (Flag MinimalDebugInfo)
-                    | str == "2" -> ParseOk [] (Flag NormalDebugInfo)
-                    | str == "3" -> ParseOk [] (Flag MaximalDebugInfo)
-                    | lstr == "false" -> ParseOk [] (Flag NoDebugInfo)
-                    | lstr == "true" -> ParseOk [] (Flag NormalDebugInfo)
-                    | otherwise -> ParseFailed (NoParse name line)
-                    where
-                      lstr = lowercase str
+              (flagElim Disp.empty (Disp.text . DebugInfoLevel.toString))
+              ( \line str _ -> case maybe NoFlag Flag (simpleParsec str) of
+                  NoFlag -> ParseFailed (NoParse name line)
+                  flag -> ParseOk [] flag
               )
 
     prefixTest name
diff --git a/cabal-install/src/Distribution/Client/ProjectConfig/Lens.hs b/cabal-install/src/Distribution/Client/ProjectConfig/Lens.hs
index 69d4e5e3aed..70f37ea38d2 100644
--- a/cabal-install/src/Distribution/Client/ProjectConfig/Lens.hs
+++ b/cabal-install/src/Distribution/Client/ProjectConfig/Lens.hs
@@ -24,9 +24,7 @@ import Distribution.PackageDescription
   ( FlagAssignment
   )
 import Distribution.Simple.Compiler
-  ( DebugInfoLevel (..)
-  , OptimisationLevel (..)
-  , PackageDBCWD
+  ( PackageDBCWD
   , ProfDetailLevel
   )
 import Distribution.Simple.InstallDirs
@@ -51,6 +49,8 @@ import Distribution.Solver.Types.Settings
   , ReorderGoals (..)
   , StrongFlags (..)
   )
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel (..))
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
 import Distribution.Types.PackageVersionConstraint
   ( PackageVersionConstraint
   )
diff --git a/cabal-install/src/Distribution/Client/ProjectConfig/Types.hs b/cabal-install/src/Distribution/Client/ProjectConfig/Types.hs
index 09d67a319e2..4c420ea7752 100644
--- a/cabal-install/src/Distribution/Client/ProjectConfig/Types.hs
+++ b/cabal-install/src/Distribution/Client/ProjectConfig/Types.hs
@@ -68,8 +68,6 @@ import Distribution.PackageDescription
 import Distribution.Simple.Compiler
   ( Compiler
   , CompilerFlavor
-  , DebugInfoLevel (..)
-  , OptimisationLevel (..)
   , PackageDBCWD
   , ProfDetailLevel
   )
@@ -98,6 +96,8 @@ import Distribution.Version
 
 import qualified Data.Map as Map
 import Distribution.Solver.Types.ProjectConfigPath (ProjectConfigPath)
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel)
+import Distribution.Types.OptimisationLevel (OptimisationLevel)
 import Distribution.Types.ParStrat
 import Distribution.Verbosity (VerbosityFlags)
 
diff --git a/cabal-install/src/Distribution/Client/ProjectOrchestration.hs b/cabal-install/src/Distribution/Client/ProjectOrchestration.hs
index cecea153f39..05b94cc0681 100644
--- a/cabal-install/src/Distribution/Client/ProjectOrchestration.hs
+++ b/cabal-install/src/Distribution/Client/ProjectOrchestration.hs
@@ -181,17 +181,13 @@ import Distribution.Client.Errors
 import Distribution.Package
 import Distribution.Simple.Command (commandShowOptions)
 import Distribution.Simple.Compiler
-  ( OptimisationLevel (..)
-  , compilerCompatVersion
+  ( compilerCompatVersion
   , compilerId
   , compilerInfo
   , showCompilerId
   )
 import Distribution.Simple.Configure (computeEffectiveProfiling)
-import Distribution.Simple.Flag
-  ( flagToMaybe
-  , fromFlagOrDefault
-  )
+import Distribution.Simple.Flag (flagToMaybe, fromFlagOrDefault)
 import Distribution.Simple.LocalBuildInfo
   ( ComponentName (..)
   , pkgComponents
@@ -215,6 +211,8 @@ import Distribution.Types.Flag
   , diffFlagAssignment
   , showFlagAssignment
   )
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
+import qualified Distribution.Types.OptimisationLevel as O
 import Distribution.Utils.NubList
   ( fromNubList
   )
@@ -1255,13 +1253,7 @@ printPlan
         "Build profile: "
           ++ unwords
             [ "-w " ++ (showCompilerId . pkgConfigCompiler) elaboratedShared
-            , "-O"
-                ++ ( case globalOptimization <> localOptimization of -- if local is not set, read global
-                      Setup.Flag NoOptimisation -> "0"
-                      Setup.Flag NormalOptimisation -> "1"
-                      Setup.Flag MaximumOptimisation -> "2"
-                      Setup.NoFlag -> "1"
-                   )
+            , "-O" ++ (O.toString . fromFlagOrDefault NormalOptimisation) (globalOptimization <> localOptimization)
             ]
           ++ "\n"
 
diff --git a/cabal-install/src/Distribution/Client/ProjectPlanning.hs b/cabal-install/src/Distribution/Client/ProjectPlanning.hs
index 6e2ef652cd8..77be4c1fa20 100644
--- a/cabal-install/src/Distribution/Client/ProjectPlanning.hs
+++ b/cabal-install/src/Distribution/Client/ProjectPlanning.hs
@@ -234,6 +234,8 @@ import qualified Data.Map as Map
 import qualified Data.Set as Set
 import Distribution.Client.Errors
 import Distribution.Solver.Types.ProjectConfigPath
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel (..))
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
 import GHC.Stack (HasCallStack)
 import System.Directory (getCurrentDirectory)
 import System.FilePath
diff --git a/cabal-install/src/Distribution/Client/ScriptUtils.hs b/cabal-install/src/Distribution/Client/ScriptUtils.hs
index 656a515a26c..d7315626da3 100644
--- a/cabal-install/src/Distribution/Client/ScriptUtils.hs
+++ b/cabal-install/src/Distribution/Client/ScriptUtils.hs
@@ -125,7 +125,6 @@ import Distribution.Parsec
 import qualified Distribution.SPDX.License as SPDX
 import Distribution.Simple.Compiler
   ( Compiler (..)
-  , OptimisationLevel (..)
   )
 import Distribution.Simple.Flag
   ( flagToMaybe
@@ -167,6 +166,7 @@ import Distribution.Types.GenericPackageDescription as GPD
   ( GenericPackageDescription (..)
   , emptyGenericPackageDescription
   )
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
 import Distribution.Types.PackageDescription
   ( PackageDescription (..)
   , emptyPackageDescription
diff --git a/cabal-install/tests/UnitTests/Distribution/Client/Configure.hs b/cabal-install/tests/UnitTests/Distribution/Client/Configure.hs
index 21a6bcabb46..07694e82214 100644
--- a/cabal-install/tests/UnitTests/Distribution/Client/Configure.hs
+++ b/cabal-install/tests/UnitTests/Distribution/Client/Configure.hs
@@ -9,9 +9,9 @@ import Distribution.Client.NixStyleOptions
 import Distribution.Client.ProjectConfig.Types
 import Distribution.Client.ProjectFlags
 import Distribution.Client.Setup
-import Distribution.Simple
 import Distribution.Simple.Flag
 import Distribution.Simple.Utils (removeFileForcibly)
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
 import Distribution.Verbosity
 import System.Directory
 import System.FilePath
diff --git a/cabal-testsuite/PackageTests/ConfigFile/InitSectionFieldsBools/cabal.out b/cabal-testsuite/PackageTests/ConfigFile/InitSectionFieldsBools/cabal.out
new file mode 100644
index 00000000000..e3dc6d7eee9
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ConfigFile/InitSectionFieldsBools/cabal.out
@@ -0,0 +1,2 @@
+# cabal user-config
+Writing default configuration to <ROOT>/cabal.dist/cabal-config
diff --git a/cabal-testsuite/PackageTests/ConfigFile/InitSectionFieldsBools/cabal.test.hs b/cabal-testsuite/PackageTests/ConfigFile/InitSectionFieldsBools/cabal.test.hs
new file mode 100644
index 00000000000..3944c7aa30d
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ConfigFile/InitSectionFieldsBools/cabal.test.hs
@@ -0,0 +1,18 @@
+import Test.Cabal.Prelude
+import Data.List (isInfixOf, groupBy)
+import Data.Function (on)
+
+main = cabalTest $ do
+  workdir <- fmap testWorkDir getTestEnv
+  let conf = workdir </> "cabal-config"
+
+  cabalG ["--config-file", conf] "user-config" ["init"]
+  confContents <- liftIO $ readFile conf
+
+  let ls = lines confContents
+      sections = groupBy ((==) `on` (== "")) ls
+      [initLs] = filter ((== "-- full-version: False") . head) sections
+      init = unlines initLs
+
+  assertBool "init section of config should contain debug-info: 0" ("debug-info: 0" `isInfixOf` init)
+  assertBool "init section of config should contain optimization: 1" ("optimization: 1" `isInfixOf` init)
diff --git a/cabal-testsuite/PackageTests/ProjectConfig/DebugInfo/cabal.out b/cabal-testsuite/PackageTests/ProjectConfig/DebugInfo/cabal.out
index 19b2a69fa0a..affd6b255e1 100644
--- a/cabal-testsuite/PackageTests/ProjectConfig/DebugInfo/cabal.out
+++ b/cabal-testsuite/PackageTests/ProjectConfig/DebugInfo/cabal.out
@@ -8,3 +8,6 @@ In order, the following would be built:
 # cabal build
 Warnings found while parsing the project file, cabal.boolean.project:
  - cabal.boolean.project:2:1: The field "debug-info" is specified more than once at positions 2:1, 3:1
+Build profile: -w ghc-<GHCVER> -O1
+In order, the following would be built:
+ - debug-info-0 (lib) (first run)
\ No newline at end of file
diff --git a/cabal-testsuite/PackageTests/ProjectConfig/DebugInfo/cabal.test.hs b/cabal-testsuite/PackageTests/ProjectConfig/DebugInfo/cabal.test.hs
index 66fb7f795f0..c0a56a3580b 100644
--- a/cabal-testsuite/PackageTests/ProjectConfig/DebugInfo/cabal.test.hs
+++ b/cabal-testsuite/PackageTests/ProjectConfig/DebugInfo/cabal.test.hs
@@ -6,7 +6,5 @@ main = cabalTest . recordMode RecordMarked $ do
   numeric <- cabal' "build" ["--project-file=cabal.numeric.project", "--dry-run"]
   assertOutputDoesNotContain errMsg numeric
 
-  boolean <- fails $ cabal' "build" ["--project-file=cabal.boolean.project", "--dry-run"]
-  -- TODO: When fixed, change this to assertOutputDoesNotContain.
-  assertOutputContains errMsg boolean
-  pure ()
+  boolean <- cabal' "build" ["--project-file=cabal.boolean.project", "--dry-run"]
+  assertOutputDoesNotContain errMsg boolean
diff --git a/cabal-testsuite/PackageTests/ProjectConfig/FlagsBools/cabal.project b/cabal-testsuite/PackageTests/ProjectConfig/FlagsBools/cabal.project
index 84e7456eccf..f405ea33f39 100644
--- a/cabal-testsuite/PackageTests/ProjectConfig/FlagsBools/cabal.project
+++ b/cabal-testsuite/PackageTests/ProjectConfig/FlagsBools/cabal.project
@@ -2,6 +2,7 @@ packages: .
 
 build-info: true
 optimization: tRuE
+debug-info: fALSe
 
 package test
   flags: -bar
diff --git a/changelog.d/11937.md b/changelog.d/11937.md
new file mode 100644
index 00000000000..63613bdd20b
--- /dev/null
+++ b/changelog.d/11937.md
@@ -0,0 +1,26 @@
+---
+synopsis: "Display -O and -g as numeric levels instead of booleans"
+packages: [Cabal, cabal-install]
+prs: 11937
+---
+
+### Cabal API Changes
+- The types `OptimisationLevel` and `DebugInfoLevel` have been moved from `Distribution.Simple.Compiler`
+to `Distribution.Types.OptimisationLevel` and `Distribution.Types.DebugInfoLevel` respectively.
+
+### Changes
+The rendering of certain boolean‑like flags has been changed to use numeric levels:
+- For the `-O` (optimisation) flag, `True` or an unspecified value now renders as `1`, while `False` renders as `0`.
+- For the `-g` (debug info) flag, `True` or an unspecified value now renders `2`, while `False` renders as `0`.
+This makes the displayed compiler flags more consistent with their internal semantics (e.g. optimisation level 1 is the default).
+- Removes the short `-g` option for the `--ghc` flag. This is now used as the short option for `--enable-debug-info`.
+```diff
+  $ cabal build --help
+...
+- -g, --ghc                      compile with GHC
++ --ghc                          compile with GHC
+...
+- --enable-debug-info[=n]        Emit debug info (n is 0--3, default is 0)
++ -g n or -gn, --enable-debug-info[=n]
++                                Emit debug info (n is 0--3, default is 0)
+```